Why isn't my Facebook chat working ?!?

Everytime I log onto Facebook, it says at the bottom of the page : " Friends online : 31 " or something, but then when I click on it, it disappears and says ' Facebook chat is currently unavailable at this time ' . And when it does occasionally reappear, once try to talk to someone, my message goes on but it says Sending ... and it doesn't actually send . It's making me really mad, and it's been like that for about 2 or 3 weeks .. Can anyone help ?!